The present invention relates to a bit-phase synchronized optical pulse stream local generator for locally generating an optical pulse stream synchronized in bit-phase with an ultrafast incoming optical signal pulse stream sent over a transmission line at a bit rate in excess of 100 Gbits/s.
With the recent fast-growing use of the Internet, data traffic is now on the increase, providing the impetus for further upgrading of large-capacity photonic networks. Time division multiplexing ranks with wavelength division multiplexing as a scheme effective in increasing the per-fiber channel capacity. The expansion of channel capacity through speedups of electronic circuit operations has now reached a level of 40 Gbits/s, where much difficulty is expected to encounter in further speedups. Optical signal processing based on a nonlinear optical effect that provides a sub-picosecond response speed is regarded as a technique capable of overcoming bandwidth limitations on electronic circuits and is now under research and development aiming at wide application to optical communication.
The optical signal processing that applies the nonlinear optical effect is to carry out switching, wavelength conversion and various other optical signal processing by timed interaction of input signal light with another ray of light (locally generated control light) in a nonlinear optical material. This technique is applicable to the generation of control light synchronized with the input signal light in all ultrafast all-optical control circuits that utilize the nonlinear optical effect, such as an all-optical time-division multiplexer, an all-optical time-division demultiplexer, an all-optical wavelength-division multiplexer and an all-optical add/drop circuit.
The optical signal processing necessitates the use of a bit-phase synchronized optical pulse stream local generator that locally generates an optical pulse stream synchronized with an optical pulse stream of a desired period in the incoming optical pulse streams. The timing accuracy necessary for the locally generated optical pulse stream becomes higher with an increase in the bit rate of the incoming optical pulse stream; for example, for bit rates of the 100-Gbit/s class, the required timing accuracy is better than one picosecond. In optical communications, since the signal light propagates usually over a long distance through an optical fiber transmission line, the timing of arrival of the signal light at the receiving end fluctuates with the expansion or shrinkage of the optical fibers used. To identify or distinguish respective bits of the received signal, it is necessary at the receiving end to extract from the received signal a clock corresponding to the timing fluctuation. The optical signal processing further requires the receiving end to prepare an optical pulse stream with the fluctuating timing.
An optical control pulse stream for processing the incoming signal pulse stream in synchronization therewith is usually generated by a mode-locked laser or similar short-pulse laser and subjected to amplification by an optical fiber amplifier and other processing, thereafter being coupled to the incoming optical signal pulse stream; in this case, the propagation delay through fairly long optical fiber components such as an optical fiber amplifier and a nonlinear pulse compression fiber readily varies (for instance, 50 ps/km/xc2x0 C.) with an ambient temperature change. It is disclosed in K. L. Hall et al., IEEE Photon. Technol. Lett., vol. 7, pp. 935-937, 1995 to use a nonlinear optical loop mirror as an all-optical bit-phase sensor to synchronize the optical control pulse stream with the incoming optical signal stream having delay fluctuations by temperature variations of the optical fiber components. Because of the use of the nonlinear loop mirror in a phase detecting part, however, the proposed loop circuit has the defects of polarization dependence and incapability of compensating for fluctuations in the propagation delay of the optical fiber used as a nonlinear optical material for the nonlinear loop mirror.
FIG. 1 depicts an example of a conventional bit-phase synchronized optical signal pulse stream local generator identified generally by 100. In Japanese Patent Application Laid-Open Gazette No. 10-209926 there is described only a synchronized clock generation part 110 composed of an optical modulator 21, a photo detector 22, frequency multipliers 23 and 32, a phase comparator 41 and a voltage-controlled oscillator 51 in the bit-phase synchronized optical signal pulse stream local generator 100 of FIG. 1. A part of incoming optical signal pulse stream SIN of a bit rate Nfa (where N is the number of multiplexed channels), which is a time-division multiplexed optical pulse stream, is provided via an optical branching device 11 to the optical modulator 21, wherein it is modulated by a signal of a frequency kfVCO generated by a k-fold frequency multiplication of the output from the voltage-controlled oscillator 51 by the frequency multiplier 23. As a result, the photo detector 22 yields an electrical signal of a downconverted frequency Nfaxe2x88x92nkfVCO. This electrical signal is applied to the phase comparator 41. The bit rate of the incoming optical signal pulse stream is as high as 100 Gbits/s, for instance, and hence it is difficult to process the optical signal pulse stream of such a high bit rate by an electronic circuit. The technique of downconverting the frequency by the optical modulator 21 as mentioned above is disclosed in, for example, Japanese Patent Application Laid-Open Gazette No. 10-65225. On the other hand, a local oscillation signal SVCO of a frequency fVCO generated by the voltage-controlled oscillator 51 is multiplied h-fold by the frequency multiplier 32 (assumed to be set at a multiplication number of h), and the resulting locally generated, multiplied signal of a frequency hfVCO is applied to the phase comparator 41 for phase comparison with the output from the photo detector 22. The voltage-controlled oscillator 51 is controlled so that the phases of the two input signals to the phase comparator 41 are locked relative to each other. The constants N, n, k and h are integers equal to or greater than 1. These constants are predetermined such that the frequencies of the two input signals to the phase comparator 41 are Nfaxe2x88x92nkfVCO=hfVCO, that is, such that the oscillation frequency of the voltage-controlled oscillator 51 is fVCO=Nfa/(nk+h) and that the value of N/(nk+h) becomes a natural number (an integer equal to or greater than 1). Accordingly, a phase error or difference signal obtained by the phase comparison is fed back as a control voltage VC to the voltage-controlled oscillator 51 to control its local oscillation frequency fVCO. In consequence, the voltage-controlled oscillator 51 is controlled so that the phases of the two input signals to the phase comparator 41 are locked relative to each other. That is, the frequency multiplier 32, the phase comparator 41 and the voltage-controlled oscillator 51 constitute a phase-locked loop PLL. The local oscillation signal SVCO output from the voltage-controlled oscillator 51 is phase-synchronized with the incoming optical signal pulse stream SIN, and drives a local optical pulse source 52. Accordingly, the local optical pulse source 52 outputs a locally generated optical pulse stream SL of a frequency fVCO=Nfa/(nk+h) that is synchronized in bit phase with the incoming optical signal pulse stream SIN.
In the conventional bit-phase synchronized optical pulse stream local generator 100 depicted in FIG. 1, a delay fluctuation occurs in the local optical pulse source 52, causing a phase fluctuation in the output pulse stream.
The local oscillation signal SVCO output from the voltage-controlled oscillator 51 ought to be synchronized in phase with the incoming optical signal pulse stream SIN under the control of the phase-locked loop by the phase comparator 41, but owing to the phase fluctuations occurring in the local optical pulse source 52 outside the phase-locked loop, the bit phase of the locally generated optical pulse stream SL is not synchronized with the incoming optical signal pulse stream SIN.
FIG. 2 is a block diagram depicting an example of the basic configuration of a typical optical time-division demultiplexer applying the nonlinear optical effect. A nonlinear optical medium 53 is connected to the output of an optical wavelength multiplexer 13 in FIG. 1, and an optical pulse stream SD synchronized with the locally generated optical pulse stream SL is extracted from the incoming time-division-multiplexed optical signal pulse stream SIN and is output. In this case, the conventional local optical pulse source 52 and the synchronized clock signal generator 110 constitute an optical control pulse stream generator, that is, the optical bit-phase synchronized pulse stream local generator 100. The synchronized clock signal generator 110 has the same configuration as shown in FIG. 1. Accordingly, it is impossible to compensate for the delay fluctuations occurring in the local optical pulse source (optical control pulse source) 52.
In the conventional optical time-division demultiplexer depicted in FIG. 2, the locally generated optical control pulse stream SL (FIG. 3B) is generated which has a repetition frequency Nfa/N (that is, fa) and is synchronized in bit phase with the incoming optical signal pulse stream SIN (FIG. 3A) of a repetition frequency Nfa (where N is a natural number), and the both pulse streams SIN and SL are multiplexed by the optical wavelength multiplexer 13 into a multiplexed optical signal SC, which is provided to the nonlinear optical medium 53.
The nonlinear optical medium 53 causes a nonlinear optical effect, such as cross-phase modulation or four wave mixing, between the signal light SIN and the control light SL. As the result of this, those pulses of the incoming optical signal pulse stream SIN which temporally coincide with the optical control pulse stream SL shown in FIG. 3B are demultiplexed directly or through an adequate optically passive component and output as the optical demultiplexed signal pulse stream SD as depicted in FIG. 3C.
As will readily be understood from the above, a timing error between the incoming optical signal pulse stream SIN and the optical control pulse stream SL will cause a failure in correct demultiplexing of an intended channel, giving rise to a serious problem that the basic function of the time-division demultiplexer cannot be fulfilled properly.
FIGS. 4A and 4B are diagram for explaining the influence of the timing error between the incoming signal pulse stream SIN and the optical control pulse stream SL. In an optical switch that utilizes the nonlinear optical effect by the optical control pulse stream SL, the temporal or evolution of a switching window (transmittance) is about the same as the temporal evolution of the optical control pulse intensity. That is, the transmittance (i.e., the signal-to-noise ratio) changes, depending on whether the incoming optical signal pulse SIN is multiplexed with the optical control pulse SL substantially at the center or lower end thereof. Accordingly, a fluctuation in the timing between the control light SL and the incoming signal light SIN causes a fluctuation in the intensity (or signal-to-noise ratio) of the demultiplexed light SD, constituting a fatal obstacle to satisfactory signal transmission. And, as the fluctuation grows, the desired optical signal pulse stream can no longer be demultiplexed and an optical signal pulse stream of an adjacent time slot is demultiplexed instead, thus bringing about a worse situation.
In some cases, the local optical pulse source (control light source) 52 is formed by, for example, an optical fiber amplifier for supercontinuum generation (Electron. Lett., vol. 34, pp. 575-576, 1998). The optical fiber amplifier for supercontinuum generation usually has a length on the order of kilometers, and its thermal expansion coefficient is around 50 ps/km/xc2x0 C.; therefore, assuming that the optical fiber amplifier is 1 km long, a temperature change of 1xc2x0 C. will cause a delay fluctuation of 50 ps. This 50-ps delay fluctuation is a time interval corresponding to, for instance, five slots in an optical signal of a 100 Gbits/s bit rate. This means that the timing error between the optical signal pulse stream SIN and the optical control pulse stream SL corresponds to five time slots.
It is therefore an object of the present invention to provide a bit-phase synchronized optical pulse stream local generator that is capable of generating an optical pulse stream synchronized in bit phase with incoming signal light even if a delay fluctuation occurs in the local optical pulse source.
According to the present invention, there is provided a bit-phase synchronized optical pulse stream local generator for generating an optical pulse stream synchronized in bit phase with an incoming optical signal pulse stream, the local generator comprising:
a voltage-controlled oscillator for generating a local oscillation signal in a phase controlled by a control voltage;
a local optical pulse source driven by the local oscillation signal to generate an optical pulse stream;
an optical branching device for branching the locally generated optical pulse stream into first and second locally generated optical pulse streams and for outputting the first locally generated optical pulse stream as an optical control pulse stream synchronized in bit phase with the incoming optical signal pulse stream;
a harmonic component local generation part supplied with the second locally generated optical pulse stream, for generating a harmonic component electrical signal that contains a harmonic component of the frequency of the second locally generated optical pulse stream;
an incoming signal component generating part supplied with the incoming optical signal pulse stream, for generating an incoming signal component electrical signal that contains its bit phase information; and
a phase comparison part supplied with the locally-generated harmonic component electrical signal and the incoming signal component electrical signal, for comparing their phases and for generating the phase error or difference between them for application as the control voltage to the voltage-controlled oscillator.
A path containing the phase comparison part, the voltage-controlled oscillator, the local optical pulse source, the optical branching device and the harmonic component local generation part constitutes a phase-locked loop for the incoming signal component electrical signal.